1. Field of the Invention
This invention, generally, relates to arrangements for detecting the movement of paper in equipment that depends on the availability of paper for its successful operation and, more particularly, to a system for detecting abnormal motion in such paper movement.
Present day equipment use paper from either rolls or, more often, from a continuous supply of fan-fold paper, for either continuous movement or for intermittent movement in a stepped manner past an operating station. Examples of such equipment include printers used with computers, teletype machines, copiers, etc.
In such use, it is important to be able to detect any movement in the paper that is not as it should be, i.e., an abnormal movement in the paper. Most of the detection devices in use today depend upon a small wheel engaging the paper frictionally for rotating in order to develop paper movement signals.
However, it has been found that, while friction wheels are effective in many instances, there are situations where a friction wheel will turn but the paper is not moving in an acceptable manner.
Examples of such abnormal paper movement include paper flutter, where the web of paper vibrates and shakes due to various causes. Usually, a small sprocket wheel with pins engage matching holes along the length of a paper web to move the paper, and these holes can become improperly spaced, even absent altogether or the paper becomes torn.
Regardless of the cause of irregular or abnormal paper movement, a variation in the movement must be detected as soon as possible, and this detection should involve a decision as to whether the variation is a temporary variation in movement or a real paper jam.
Printers of the type used with computers usually use fan-fold paper that forms a continuous web which is moved intermittently in a step by step manner, and therefore, the paper web is moved by the distance of one or more lines for a printing head to execute a line of type at a time. When an undesirable condition is detected, a decision must be made quickly, simply and, preferably, automatically, whether or not to shut down the printer operation.
2. Description of the Prior Art
Prior devices sense paper movement because a friction wheel generates a signal that says there is paper movement, but it cannot say whether such movement is correct.
An example of the prior devices is U.S. Pat. No. 3,917,142 granted Nov. 4, 1975 to Guarderas that describes a monitoring arrangement using a friction wheel and a disc with slots to interrupt the optical path of light producing a pulse train to reset a counter after each paper feed command. When a paper jam occurs, no reset pulse is given to the counter, and after a number of counts, it generates a signal to terminate operations.
U.S. Pat. No. 3,949,856 to Ulber et al., granted Apr. 13, 1976, describes a system using two sources of signals, one source generates signals in response to movement of a paper feed wheel, and a second source generates signals in response to a friction wheel engaging the paper. The two signals are counted and compared, and if such comparison is not within expected limits, an indication is given.
While the prior systems at first appearance have similarities with the invention, they differ in material respects, and these differences will become more apparent as the description proceeds.